This application claims the priority of Korean Patent Application No. 2003-64719, filed on Sep. 18, 2003, in the Korean Intellectual Property Office, the disclosure of which is incorporated herein in its entirety by reference.
1. Field of the Invention
The present invention relates to a technique of miniaturizing a magneto-optical storage device, and more particularly, to a planar waveguide-type polarization detection module, which is very important for realizing an integrated optical pickup head (IOPH) into a magneto-optical pickup head.
2. Description of the Related Art
A typical magneto-optical storage device reads data from a recording medium by measuring a voltage/current signal that reflects polarization rotation rate variations, which are triggered by reflections of beams focused on the recording medium according to the Kerr effect. In order to detect such polarization rotation rate variations, a polarization detection module, which includes a polarization beam splitter and a photo diode, is generally used.
FIG. 1 illustrates an example of a conventional polarization detection module for a magneto-optical storage device. Referring to FIG. 1, the polarization detection module 20 includes an object lens 11, which is located close to a disk 10, and a partial polarization beam splitter 21, which is placed between the object lens 11 and a light source 13. A half waveplate 22 and a polarization beam splitter 23 are placed over a splitting path of the partial polarization beam splitter 21. First and second photo diodes 24 and 25 are located along a beam transmission path and a beam reflection path, respectively, of the polarization beam splitter 23. The disk 10 receives a beam from the light source 13 and then reflects the beam. The partial polarization beam splitter 21 polarizes the original beam emitted from the light source 13 and reflects the beam reflected from the disk 10 to the half waveplate 22. The half waveplate 22 has a certain-directional principal axis. Light passing through the half waveplate 22 enters the polarization beam splitter 23 and then is split into a p-wave and an s-wave such that the p-wave and the s-wave enter the first and second photo diodes 24 and 25, respectively. Each of the first and second photo diodes 24 and 25 generates current, the amount of which corresponds to the amount of light input thereinto. The current generated by each of the first and second photo diodes 24 and 25 is transmitted to a driving circuit (not shown) that extracts data or processes servo error signals.
However, there is a clear limit in integrating the above-mentioned elements of the conventional polarization detection module 20 into one body because each of the elements of the conventional polarization detection module 20 has a relatively large volume. If the polarization beam splitter 23 of the polarization detection module 20 could be manufactured into a waveguide-type polarization detection module, it would be easier to integrate the elements of the polarization detection module 20 into one body.
For example, an optical pickup head for a CD/CDR/CDRW, which is manufactured by patterning a focusing grating on a planar waveguide, has been reported by S. Ura et al., in “An Integrated-Optic Disk Pickup Device” (J. Lightware Technol. Vol. 4, pp. 913–918, 1986). FIG. 2 illustrates an example of the optical pickup head. Referring to FIG. 2, an optical pickup head 30 includes a planar waveguide 32, which is formed on a substrate 31, and a laser diode 33, a photo diode 34, a beam splitter 35, and a focusing grating 36, which are integrated into one body on the planar waveguide 32. In FIG. 2, reference numeral 37 represents a focusing grating coupler (FGC). The optical pickup head 30 detects an optical signal that indicates a difference between amounts of light reflected from a recording medium, i.e., an optical disk. Therefore, it is possible to manufacture an optical pickup head 30 with a small form factor by integrating all the necessary elements for detecting an optical signal into one body on the planar waveguide 32 with the help of the focusing grating 36.
As an application of the above-described technique of manufacturing an optical pickup head, a polarization detection module, in which a grating is carved on a planar waveguide so as to realize a magneto-optical pickup head, has been disclosed by S. Ura et al., in “Focusing Grating Couplers for Polarization Detection” (J. Lightwave Technol. Vol 6, pp. 1028–1033, 1988). FIG. 3A illustrates an example of a conventional planar waveguide-type polarization detection module. Referring to FIG. 3A, a polarization detection module 40 includes a planar waveguide 42, which is formed on a substrate 41, and a photo diode 43 and an input coupling focusing grating 44, which are integrated into one body on the planar waveguide 42. The polarization detection module 40 detects a polarization signal and a focusing/tracking error signal by coupling light input thereinto to the frequency of the input coupling focusing grating. The frequency of the input coupling focusing grating varies depending on whether a polarization mode is a transverse electric (TE) mode or a transverse magnetic (TM) mode. In FIG. 3A, reference numeral 45 represents a focusing grating coupler.
Only when it is possible to monolithically integrate all necessary elements of a magneto-optical pickup head into one body, it is possible to manufacture the magneto-optical pickup head in large quantities and reduce the manufacturing costs. However, it is impossible to monolithically integrate the polarization detection module 40 of FIG. 3A and a focusing waveguide grating coupler into one body. Therefore, in order to realize a magneto-optical pickup head using the polarization detection module 40, the polarization detection module 40 should be aligned in such a manner that the polarization detection module 40 and the optical pickup head 30 of FIG. 2 overlap each other, as shown in FIG. 3B. However, it is very difficult to make the polarization detection module 40 and the optical pickup head 30 overlap each other.
The polarization detection module 40 of FIG. 3A couples a beam to the planar waveguide 42 using the input coupling focusing grating 44 having a predetermined frequency, and thus it generally has a very low coupling efficiency. Moreover, if the polarization detection module 40 and the optical pickup head 30 are integrated so that they overlap each other, as shown in FIG. 3B, the amount of light focused on a magneto-optical disk 51 may be undesirably small because of the higher-order refraction effect.